In electroanalytical measurements, a sensing electrode is used in combination with a reference electrode. When both electrodes are immersed in a solution of an ionic salt, a potential difference is established between the electrodes whose magnitude is a function of the concentration of the dissolved ionic species.
In measuring silver ion activity in a halide solution or dispersion, a bare silver electrode will respond to the silver ion activity but will respond relatively slowly to changes in halide ion activity. The response can be increased by coating the surface of the silver electrode with a layer of the silver halide. The silver ion activity at the electrode surface is, therefore, directly affected by changes in halide ion activity in accordance with the solubility product of the silver halide.
Potentiometric methods are known for measuring the equilibrium silver ion activity in mixed silver halide solutions and dispersions. Two methods have been used in process control environments where the halide ion is in excess and the silver ion activity is relatively small.
One method employs a silver electrode which has been coated with a layer of silver halide either chemically or electrochemically. This electrode will respond to the particular halide coated and hence give the silver ion activity in equilibrium with the particular halide coated. Good precision, accuracy and a fast response time are obtained in solutions or dispersions consisting of the halide ion coated. However, in solutions or dispersions of mixed silver halide, the electrode coating is chemically changed to reflect the new ratio of halide ions and takes several minutes to settle to the correct value for the equilibrium silver activity. This type of electrode is unsuitable for process control of mixed halide solutions or dispersions.
A second method uses a membrane of silver halide where the electrode contact is either directly made to the rear of the membrane or is made via a filling solution containing an electrode with a fixed potential. A membrane or pellet is prepared from either particles of a single silver halide or from a mixture containing two or more halides and possibly silver sulphide. The membrane electrode has a smaller surface area to be chemically changed and therefore responds slightly more quickly than the previously described electrode. However, due to the mixed nature of the membrane and the method of construction, the value obtained is not always theoretically predictable. For this reason, careful calibration of the membrane electrode is required. Hence, this electrode is also not recommended for use in the process control of mixed halide solutions or dispersions.